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Topic Review (Newest First)

09-27-2008 07:29 AM

trantor12020

I put in the new mixing elbow today. What a disappointment. There's no increase in load rpm. Infact the exhaust creates louder popping sound compared to the old orificed mixing elbow. Makes me wonder if the orifice is intended and not result of corrosion. So no re-pitching of my prop.

09-25-2008 03:18 PM

Valiente

I am repropping from an 18 x 13 fixed three-bladed to a 19 x 15 (nominal) feathering four-bladed VariProp because I hit six knots in flat seas at 2,100 rpm or so on a 52 hp Westerbeke and I think my power curve would be better swinging a slightly beefier prop. I don't care if I lose a touch of speed, but for maneuvering under power, I believe the new prop will provide more stopping power in tight quarters. This has been very much the experience of a friend with a big steel boat even heavier than mine who put an 18 inch AutoProp off a 35 HP (yes, too small) Volvo...his boat started parking like a minivan.

Also, I can adjust the pitch if I find I am taxing the engine overmuch, but I don't think that's going to be the case.

I think having the engine able to do 3600-3800rpm on existing prop is comforting and doesn't mean I'll run at that speed all the time or at all.

You seemed so enthused at the prospect of being able to that I probably got the wrong impression.

Sorry.

09-24-2008 08:19 PM

trantor12020

I think having the engine able to do 3600-3800rpm on existing prop is comforting and doesn't mean I'll run at that speed all the time or at all. Mostly I do betw 2600-3000rpm but with some white smoke (and doesn't smell or look like steam, nor do I burnt luboil). Also with max 3600rpm in gear, there is scope for me to re-pitch my prop currently 14"Dx6"P 3-Blade Campbell Sailor by an inch or two. I've an 18HP 2GM20F on a 8500lbs displ boat. Its supposed to be sufficient but at times, I've to struggle against tide and also usually last in a convoy (when no wind).

Going to max rpm for a brief period is a way of making sure everything in the propulsion system is working properly. If you can't get there you may have too much prop pitch or a problem with the motor. Also, many mechanics suggest running at wide open throttle for 10 minutes a few times a season to help keep carbon build up to a minimum. And, if you plan on selling the boat a surveyor will likely want to run at WOT and see of the engine reaches the max rated rpm.

I'm not suggesting that running at WOT should be strictly avoided. It's really just that the OP seemed to express exhuberant glee at the prospect (quote: "I hope I can get to at least 3800rpm" on an engine that has that as it's max governed speed).

Without wanting to put too fine a point on it . . . .

Problems with the propulsion system will manifest themselves soon enough and if your prop pitch is wrong you should know that without having to regularly run the engine at WOT

If your engine is running efficiently and is mostly used at the optimum spot in the fuel consumption curve, there shouldn't be any carbon build-up and IMHO mechanics who subscribe to that philisophy are cowboys. You will rarely find a responsible truck operator running his engines at max RPM "to blow the carbon out". Remember there is a big difference between WOT and max RPM.

I have never found any justification for running at max RPM unless I need strong emergency propulsion. I guess if this is what the OP was suggesting then I've got it wrong.

09-24-2008 05:07 PM

SteveInMD

Going to max rpm for a brief period is a way of making sure everything in the propulsion system is working properly. If you can't get there you may have too much prop pitch or a problem with the motor. Also, many mechanics suggest running at wide open throttle for 10 minutes a few times a season to help keep carbon build up to a minimum. And, if you plan on selling the boat a surveyor will likely want to run at WOT and see of the engine reaches the max rated rpm.

Sooo..., if I put in this new Mixing Elbow, I hope to get at least 3600rpm !!

Why would you want to do that?!?!?!

Look what happens to your torque curve at the top of the rev range (second graph)

Way more important, look at what happens to your fuel consumption (top graph) when you get up there!!! At the cost of fuel today, I would stay well away from 3600 rpm and that's not even discussing engine durability.

BTW, this engine curve is a Yanmar 3YM30

I'm also wondering how the water aperture in a mixing elbow increases RPM availability. It obviously does . Or are we discussing an exhaust outlet that has been reduced to 10mm ??

09-24-2008 06:37 AM

trantor12020

Ok my parts has arrived and I checked the exhaust hole. It is BIG. Much bigger (at least 5 times) than the 10mm orifice I have in the old ones.
After I cleaned up the old elbow, my rpm increased from 3000 to 3200rpm.
Now after I've antifoul the hull and cleaned up the prop & shaft, I can get 3400rpm. Sooo..., if I put in this new Mixing Elbow, I hope to get at least 3600rpm !!

09-09-2008 06:52 AM

trantor12020

Ok thanks for the info. I just fitted the repaired parts in and the engine runs as before. So the exhaust hole should actually be larger? hmmm, maybe that's why I can't bring up my rpm pass 3100 under load. Can't wait to get my hands on the new parts. BTW, the Mixing Elbow has reverse (left-handed) thread if anyone is interested to know. hence the joint has 1 side LH thread and the other RH thread. Sharks! my Yanmar dealer didn't even know this when I asked.

09-08-2008 09:04 PM

rireefguy

had the same problem... was able to poke it enough to get it going while I waited for the part to arrive. the carbon build up was pretty hard, but the real problem was get something up in there. the hole I was able to open up was the size of a drinking straw and that was enough to get the engine running. The new one had 10x larger opening. you should be fine till the part arrives.

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